Washing bars can be classified into soap bars, mixed active bars containing a significant proportion of soap and thirdly synthetic detergent bars containing only a small proportion of soap or none at all.
Conventional soap bars comprise a large proportion, typically 60-80% by weight, of fatty acid soap. Fatty acid soaps are selected to provide a balance of soluble and insoluble soaps which provide the required functional properties as regards lather formation and bar structure. Conventional soap bars are manufactured by milling, plodding and stamping a semi-solid mass of soap and other components.
Bars are known which contain a mixture of soap and synthetic detergent where the amount of soap may be less than the amount of synthetic detergent but is nevertheless still a significant contributor to the content of the bar. In such bars, as in conventional soap bars, the content of soap, especially the insoluble soap, contributes to the structure and physical properties of the bar.
The third category is synthetic detergent bars, often known as "Syndet" bars, in which there is no soap or only a small amount and the detergent active is mostly or wholly a synthetic, non-soap, detergent. Generally such bars contain a substantial proportion of material which is not a detergent and which serves to give structure to the bar. Such "structurants" are normally water-insoluble and include such materials as starch and kaolin. The bars frequently also contain a plasticiser: known plasticisers include stearic acid and cetyl alcohol. Known surfactants for Syndet bars include primary alkyl sulphates, alkyl ether sulphates, betaines, sarcosinates, sulphosuccinates and isethionates. These syndet bars containing no soap or only a small proportion of soap are traditionally produced by energetic working of a physical mix of structurant, plasticiser and surfactant, i.e., both the soluble and insoluble components, in a high shear mixer to an end point at which the product is not gritty. The mix is then formed into `syndet` bars.
The known process has several disadvantages in that the physical mixing step is performed batchwise and requires an energetic mixer.
We have now found that by adopting a novel composition, syndet bars may be produced by a process which dispenses with the known energetic working step.
In contrast with prior compositions and processes, the invention relies on ingredients which are molten at conveniently accessible temperatures but which are above the temperatures normally encountered during use of "Syndet" bars. As a result the necessary intimate mixing of the ingredients of the bar can be accomplished by simple mixing while the bar composition is liquid rather than by relying on energetic working to achieve intimate mixing of a mixture of solids.
The present invention further recognizes that it is only a specific class of water-soluble structurants, i.e., those having defined minimum melting points, which can function to partially replace hydrophobic fatty acid structurants normally used in bar structuring.
If the melting temperature is too low, the composition will be too "liquidy", extruded bar product will be extremely soft and during refining stage, rather than typical "noodles", large sticky balls will typically form. Yield stress measurements show extremely soft and essentially, from a consumer perspective, useless bar.
If, on the other hand, melting point were too high, the bars would be too sticky to process, have low lather and low dissolution.
In short, the water-soluble structurant must be chosen precisely so as to be not too liquidy, so as to be hard enough to process well, yet not be so hard as to form sticky product which will clog machinery and inhibit processing.
It has never been previously recognized that large amounts (i.e., 20% or greater) of specific water-soluble material (e.g., alkylene oxides) could be used for this purpose because there was no recognition that minimum melting point (i.e., MW) was required. Water soluble materials such as alkylene glycols in the art have traditionally been viewed as "moisturizing" ingredients and the materials used would be generally perceived by the art to be liquidy and to not process well.
U.S. Pat. No. 4,812,253 to Small et al., for example, discloses a composition comprising surfactant (component (a) of the subject invention), water-insoluble structurant such as fatty acid (component (c)) and water (component (d)). Although Small et al. mentions that polyalkylene glycol can be used as "moisturizer/emollient" at levels of 10-40% by wt., there is nothing in this reference teaching or suggesting the melting point or MW be above certain minimum levels (i.e., 40.degree. C. and up, preferably 47.degree.-100.degree. C., more preferably 50.degree. to 100.degree. C.).
Indeed, there are no examples of such "moisturizer" at all and preferred moisturizers are said to be coco and tallow fatty acids. As noted, previous art would not have used high levels of alkylene oxides as structurants because they would have believed the bar was unprocessable or, if processable, would create soft, mushy bars of very low yield strength. Nothing in this or any other reference would have motivated the inclusion of specifically defined water-soluble structurants of the invention.